Chapter V: Preface: ─────── (5)

179. GLEN TILT MARBLE _is of white or grey colour, and veined or spotted with yellow or green; some specimens are nearly white_. The granulations are peculiarly large; and, in its aspect and composition, the Glen Tilt has great general resemblance to the Pentelic marble (143). This marble has of late attracted the notice of the Duke of Athol, through the suggestion of Dr. Macculloch; and chimney─pieces of it have since been made. It is obtained from a valley of the same name in the county of Perth.

180. BLAIRGOWRIE MARBLE.—A few miles from Blairgowrie, in Perthshire, there is an excellent granulated broad─bedded marble, _of sugar─loaf texture, and as white as the finest statuary marble_. It may be easily raised in blocks and in slabs of great size, perfectly free from blemishes. This marble is supposed to be well adapted for ornamental architecture, but its large sparry texture renders it unfit for the sculptor.

181. GLENAVON MARBLE _is of white colour, with large granular concretions_, somewhat like spangles, and as large as the scales of fishes. This is a valuable kind; but its situation in the forest of Glenavon, on the property of the Duke of Gordon, is remote and difficult of access.

182. BALLICHULISH MARBLE.—On the north side of the ferry of Ballichulish, in Lochaber, there is a rock of marble, of beautiful _ash─grey colour_, and of a fine, regular, and uniform grain, which is capable of being wrought into blocks or slabs of any size, and is susceptible of a fine polish. This marble is finely sprinkled throughout with grains and specks of pyrites (236), and with grains and specks of a beautiful lead ore, which to the eye appears to be rich in silver. If used for ornamental purposes, it would be a bright and beautiful metallic marble.

183. BLAIRMACHYLDACH MARBLE.—In the bed of a river, at the farm of Blairmachyldach, about three miles south of Fort William, is a singular marble, consisting of _a black ground, flowered with white_. It is of fine close grain, but not very hard. The flowering in it is light, and beautiful, like fine needle─work, or rather resembling the frosty fret─work upon glass windows, in a winter morning.

The cutting and polishing of marble appear to have been performed by the ancients nearly in the same manner as it is with us. In polishing, the first substance employed is a sharp, coarse─grained sand. Afterwards a finer sand is used, then emery (58) in different degrees of fineness. These are followed by a red powder called tripoli (119): and the last polish is given with putty.

184. _BLACK MARBLE is a species of limestone, of uniform black colour, and easily distinguishable, by an excessively disagreeable smell, which is emitted on rubbing two pieces of it together, or striking it with a hammer._

Few minerals are susceptible of a more beautiful polish than this. It is consequently much used for chimney─pieces, small columns, vases, and other ornamental work. There are two quarries of black marble near Bakewell, in Derbyshire: and it is manufactured to a considerable extent by Messrs. Brown and Co. at Derby, who have fixed up in their ware─rooms a large slab of it as a looking─glass.

By the ancients it was much prized. Marcus Scaurus is said to have ornamented his palace with columns of black marble, each thirty─eight feet high; and many of the monuments of ancient Persepolis were executed in it. M. D’Avejan, Bishop of Alais, used a kind of black marble for paving the apartments of his palace; but the friction and heat rendered it so fetid that his successors were compelled to substitute another species of stone in its place.—The pavements, however, of many churches, and of the porticos of several galleries, on the Continent, are of black marble.

185. _CALCAREOUS ALABASTER is a species of limestone of somewhat whitish or yellowish colour, translucent, and internally splendent or shining._

_It is nearly a pure carbonat of lime; and occurs in masses, hanging, like immense icicles, from the roofs of lime─stone caverns, and also coating the sides of such caverns._

The formation of this substance is deserving of notice. The water which oozes through the crevices of limestone rocks, becomes strongly impregnated with minute particles of lime. This water, when it has reached the roof or side of a cavern, is generally suspended, for a considerable time, before a drop of sufficient size to fall by its own weight is formed. In the interval which thus elapses, some of the particles of lime are separated from the water, owing to the escape of the carbonic acid (26), and adhere to the roof. In this manner successive particles are separated, and are attached to each other, until what is called a _stalactite_, having somewhat the appearance of an icicle, is formed. These stalactites are sometimes solid, having a lamellar structure; sometimes of a fibrous texture, radiating from the centre to the circumference, as may be observed when they are broken; and sometimes hollow. If the water collects and drops too rapidly to allow time for the formation of a stalactite, it falls upon the floor, and there forms an irregular lump of alabaster, which has the name of _stalagmite_. In some caverns, the separation of the calcareous matter takes place both at the roof and on the floor; and, in course of time, the substance upon each increasing, they meet, and form pillars, sometimes of great magnitude.

Caverns of this kind occur in almost every country. Those of Derbyshire are well known; but the most celebrated stalactitic cave in the world is that of Antiparos, in the Grecian Archipelago.

The kind of limestone formed in the above manner is what the ancients generally denominated _alabaster_. It was employed by them for the same purposes as marble, was cut into tables, columns, vases, and sometimes even into statues. They also used it in the manufacture of vases or boxes for containing unguents. It is supposed to have been a vessel formed of this stone that is mentioned in the Gospel of St. Matthew, where it is said there came unto our Saviour “a woman having an alabaster box of precious ointment.” In the National Museum at Paris there is a colossal figure of an Egyptian deity, which is cut in a kind of alabaster brought from the mountains between the Nile and the Red Sea.

186. _TUFA, or INCRUSTING CARBONAT OF LIME, is a calcareous substance deposited by such water as is impregnated with lime._

_It clothes, with a stony coat, the smaller branches of trees, leaves, moss, plants, and other substances; and thus preserves them from decay, by protecting them from the action of the atmosphere._

Most of the substances termed by the common people _petrifactions_ belong to this kind of lime. They are, however, merely covered with, and by no means converted into stone.

The dropping well at Knaresborough, in Yorkshire, is particularly celebrated for them. An overhanging rock, several yards in depth, has been gradually formed of the calcareous matter which the water holds in solution; and, from this rock, it incessantly drops into the basin below. The persons who have the care of the place constantly keep these petrified articles for sale. Even old wigs and hair brooms are subjected to the powers of the water, to furnish subjects for attraction to the visitors. There are other springs of this description in Oxfordshire and Somersetshire, and particularly at Matlock, in Derbyshire. We are informed that at Dalton, on the south side of Mendip, the workmen not unfrequently discover large pieces of oak enveloped in blocks of stone which are four or five tons in weight.

Blocks of tufa are, in some countries, cut and used for building stones; and this substance, when burned, becomes an excellent lime. Pieces of it are sometimes hollowed, and used as filtering stones.

In the British Museum there is a human skull completely incrusted with stone, which was found in the river Tiber.

The warm baths of Hungary are often so thickly coated at the sides and bottom with tufa, that, during certain intervals, it actually fills up the tubes and canals through which they are supplied. The _fur in teakettles_ is a somewhat similar deposit from water in boiling.

187. _PORTLAND STONE, BATH STONE, KETTON STONE, are different kinds of limestone; and, of a texture so hard and compact as to be used in building._

_They have their names from the places where they are respectively found, in Portland Island, near Bath, and at Ketton, in the county of Rutland._

Of Ketton stone several of the colleges in Cambridge are built. Its grain has a singular resemblance to the petrified roe of a fish, whence also it is sometimes called _roestone_. The bridges, St. Paul’s Cathedral, the Monument, and nearly all the buildings of late date in London, are constructed of Portland stone.

Some of these kinds of stone, when first dug out of the quarry, are so soft that they are readily worked into any form which use or ornament may require. This is owing to the moisture with which they are naturally impregnated; but when they once become hardened, by exposure to the sun and air, they are extremely firm and solid. On the contrary, other kinds of limestone that are used for buildings imbibe and retain the moisture of the atmosphere, in consequence of which they burst or are crumbled by frost.

We are informed that Portland stone was first used in London in the reign of James the First, that monarch, by the advice of his architects, having employed it in the construction of the banquetting house at Whitehall. After the great fire in London, it was brought into general use by Sir Christopher Wren.

188. _MARL is a combination of clay, silex (76), and lime: and is denominated calcareous, argillaceous, or siliceous, as the lime, clay, or silex, is most abundant._

The calcareous part of marl is frequently composed of shells, whence it frequently has the name of _shell marl_; and where these are predominant, it affords an excellent manure for sandy, dry, gravelly, or light lands. Marl likewise produces very beneficial effects on mossy and clayey soils; and these effects, where it has been properly applied, have been observable for twelve or fourteen years. Some kinds of marl that contain but a small portion of lime have been successfully used in the manufacture of earthenware.

This mineral is usually found at the depth of from five to nine feet beneath the surface of the ground, and deposited between beds of clay and sand. It is dug out with spades; and, in the digging of it, in Ireland, the workmen not unfrequently meet with the horns of deer and other curious fossils.

The usual mode by which persons, generally unacquainted with minerals, distinguish this from other clayey substances, is, to break a small piece of dry marl into a glass of vinegar. If it be marl it will immediately dissolve with considerable effervescence; and the briskness of the effervescence will be in proportion to the quantity of lime which it contains.

189. _FLORENCE MARBLE is a kind of indurated or hardened marl, and is remarkable for presenting, when polished, the appearance of ruined edifices or rocks._

This kind of marble is never used in architecture. Little slabs of it are cut for Mosaic work, and to be framed like pictures; and the latter, when of considerable dimensions, are sometimes purchased at a high price. If held at a distance from the eye, an inexperienced observer might mistake a slab of Florence marble for a drawing in bistre. Here, observes a French writer, we remark a shattered Gothic castle, there the mouldering fragments of a cathedral; in one part ruined walls, and in another shattered bastions and towers. But, when we approach the picture, the illusion vanishes, and those imaginary figures which, at a distance, appeared to be so correctly drawn, become changed into irregular spots, lines, and shades, which present nothing distinct to the view.

190. COTTAM MARBLE, which, when cut and polished, also exhibits the appearance of a landscape, is a kind of compact marl. It has its name from being found at Cottam, near Bristol.

191. _LIAS, or CALP is a kind of limestone of bluish black, or greyish blue colour, and composed chiefly of lime, silex (76), clay, and oxide of iron (21)._

This stone, when burned, forms a cement which has the property of setting very strongly under water. It has also, of late years, been employed in a manner which merits particular notice, for the multiplying of copies of drawings and penmanship. A drawing is made on prepared paper with a peculiar kind of ink. A slab of lias, about an inch thick, is then heated; the drawing is placed upon it, and both are passed through a rolling press. The paper is afterwards wetted, and washed from the stone; but the ink, being of a gummy or glutinous quality, becomes in part absorbed by the stone, and remains. The stone is then ready for the printer. Previously to taking off each impression, the stone is wetted with a sponge; fresh ink (which is said somewhat to resemble printers’ ink, and is put on with a ball similar to that used by letter─press printers) is then applied. This is prevented, by the water, from adhering to any part except to the ink that had been absorbed, by the stone, from the paper on which the drawing was originally made. Paper is then placed on the stone, both are passed through a rolling press as before, and a perfect impression of the drawing is made upon the paper.

This art has been practised in Germany with great success; and with the difference only of the original drawing being made upon the stone instead of paper. Many beautiful specimens of drawings, taken from slabs of lias, may be seen in this country. It is said that copies of military drawings and orders were, to a very large amount, multiplied by this means at the headquarters of the armies lately employed on the Continent.

An artificial composition is sometimes used instead of lias.

Considerable quarries of this stone are wrought in Germany. It is also found at Leixlip, near Dublin; in beds at Aberthaw, in Glamorganshire; in Dorsetshire, and near Bath.

SULPHAT OF LIME.

192. _ALABASTER, or GYPSUM, is a kind of sulphat of lime, or of lime in combination with sulphuric acid (24), which has a shivery and glittering texture; and is of white colour tinged with grey or red, and sometimes striped, veined, or spotted. When crystallized, the primitive form of its crystals is a regular four─sided prism_ (Pl. II, Fig. 14.)

Being considerably softer than marble, this mineral is not capable of receiving a good polish. From this circumstance it is, however, the more easily worked. It is manufactured into chimney─pieces, columns, busts, ornamental vases, and lamps; the latter of which transmit a soft and pleasing light. Such is sometimes the transparency of alabaster, that it has been employed for windows; and, at Florence, there is now a church which receives its light through the medium of this substance.

The ancients, though acquainted with the art of making glass, had not attained the knowledge of reducing it into thin transparent plates; and frequently employed alabaster for windows. Of this stone the Temple of Fortune, which was built by order of the Emperor Nero, was erected. It had no windows whatever, and received only a soft kind of light through its walls; appearing rather as if the light issued from the interior, than that it was admitted from without.

The hot springs of St. Philip, which supply the baths of Tuscany, are so strongly impregnated with alabaster, that artists take advantage of this to obtain impressions of bas─reliefs, by merely exposing their moulds to a current of the water until they become filled with the earthy deposit. These impressions, when taken out, are found to be as hard as marble, and are very beautiful. There are, in the British Museum, some casts of medals formed from the water of these springs.

When alabaster is heated, it falls into a soft white powder, which, on being mixed with water, absorbs it so rapidly, that if it be formed into a paste, it dries and becomes hard in a few minutes. In this state it is called _plaster of Paris_; and is employed for the making of statues, casts, and other ornamental work, which, though of a beautiful white colour, are very brittle. When mixed with coloured gummy or glutinous substances, it yields plasters of different hues, and has the name of _stucco_; and, in this state, is used for lining the walls and ceilings of rooms. This plaster is much in request in the northern counties of England, for the floors of dairies, store─rooms, granaries, and other apartments; and, when properly formed, it constitutes a very smooth and durable flooring.

The fine white varieties of gypsum are used as an ingredient in the composition of earthenware and porcelain; and the glaze, or enamel, with which porcelain is covered, has the purest gypsum for one of its ingredients. Of late years this mineral has been advantageously employed as a manure for fertilizing the soil.

Gypsum is found in Cheshire and Derbyshire, as well as in several parts of the Continent. That which is imported into this country from Italy and Spain is considered the best.

193. FIBROUS GYPSUM.—There is a variety of gypsum which has a somewhat fibrous appearance, and which, when cut in a convex form, and polished, reflects a light not much unlike that of the cats─eye (86). Hence it is sometimes sold to ignorant persons for that stone. It has also been imposed upon purchasers for the gem called moonstone (113). Fibrous gypsum is cut into ear─pendants, crosses, beads for necklaces, and other female ornaments; but its softness is such as to allow of its being easily injured both by dirt and friction.

FLUAT OF LIME.

194. _FLUOR SPAR, or DERBYSHIRE SPAR, is a mineral formed by the combination of lime with fluoric acid (27)._

_It sometimes occurs in a massive, and sometimes in a crystallized state; the primitive form of its crystals being a regular octohedron_ (Pl. II, Fig. 5). _Its colour is usually bluish, green, yellow, whitish, or a mixture of some of these._

When heated, this substance cracks, and shines brightly in the dark. But if kept hot for some time, it ceases to be luminous, and this property cannot be restored to it. If also two pieces be rubbed strongly together, they become luminous in the dark.

From this spar are made several kinds of ornamental vases of considerable size, columns, and toys, which, from being extremely varied in their colours and appearance, and admitting of a high polish, are very beautiful. When a piece of fluor spar is to be wrought into a vase, or any similar article, it is first carved with a mallet and chisel into a somewhat spherical form. It is then fixed to a turner’s lathe, and, with great care, is formed into the shape that is required. When this is complete, it has to be polished, which is done first with gritstone and pumice (108), and lastly with emery (58) and putty. The lathes formerly in use were worked by the foot; but those now adopted are worked by machinery, the advantage of the more steady motion of which has been that ornaments of much more delicate structure can now be formed than before. The manufacture of articles from fluor spar gives employment to a great number of industrious families in Derbyshire. This mineral occurs in several parts of that county, where it has the name of _Blue John_, and where it is obtained from caverns at a considerable depth beneath the surface of the earth. It is also found in various countries both of the European and American continents.

The acid produced from fluor spar is called _fluoric acid_ (27), and has the peculiar property of corroding glass and flint, and consequently cannot be kept in glass bottles. Artists, by means of fluoric acid, are enabled to etch on glass, in the same manner as, with aqua fortis (nitric acid), they do on copper. The process is sufficiently simple. The glass is first a little heated, for the purpose of covering it thinly over with wax; then, with a needle or other fine point the drawing is to be made, by cutting through the wax to the surface of the glass. The edges are next to have a little wall of wax raised upon them. This done, the glass must be placed in an horizontal position, and sifted over with fluor finely pounded; and lastly, a mixture of one part of spirit of vitriol or sulphuric acid (24) with two or three parts of water is to be poured gently upon it. The acid will be prevented from running off by the wax; and, in the course of a little while, if these be cleared away, the glass will be found corroded in all the lines along which the needle passed.

The mode of obtaining fluoric acid for chemical purposes is, by pouring sulphuric acid upon powdered spar in a leaden retort, and applying to it a gentle heat. This acid should be used with great caution; for, when applied to the skin, it instantly disorganizes it, and produces very painful sores.

BARYTES FAMILY.

195. These minerals are sometimes called _ponderous earths_, and have their name from a Greek word signifying _heavy_. They comprehend all the combinations of barytes with acids.

When purified, they form a greyish white, porous substance, which is easily reducible to powder; has no perceptible smell, but has a harsh and more burning taste than lime, and changes the blues of vegetable colours to green.

Although barytes is one of the most useful chemical tests that we are acquainted with, it is not much employed in the arts, because, when purified, it is found too expensive. It is capable of being made into a very tenacious cement; and painters use a preparation that is made from it as a white colour which will not change. This is sold in the shops under the name of “Hume’s permanent white.” Barytes taken into the stomach proves a virulent poison; yet a preparation of it is used in medicine, and particularly for the removal of scrophulous complaints. When finely pounded and mixed with oatmeal, _carbonat of barytes_ has been found an efficacious poison for rats.

196. _SULPHAT of BARYTES is a mineral formed by the combination of sulphuric acid (24) with barytes._

_It sometimes occurs in a state of powder, frequently in shapeless masses, and often crystallized: the primitive form of its crystals being a four─sided prism. It is not soluble in any other than sulphuric acid._

With us sulphat of barytes is of no use in the arts. The Chinese, however, employ it as an ingredient in the composition of porcelain; and it is said to form a good manure for clover fields.

_The_ BOLOGNA PHOSPHORUS, _or_ BONONIAN STONE, _a very remarkable kind of barytes, has its name from being found near Bologna in Italy._ This substance, when detached, is usually observed in roundish, flat, kidney─shaped pieces, from about the size of a walnut to that of an orange, which have a shining and somewhat fibrous texture within.

When the outer coat of this stone is washed away by heavy rains, it has sometimes the appearance of burnished silver. An Italian shoemaker, in the year 1630, deceived by this appearance, carried home several pieces, hoping, by means of fire, to extract silver from them. But at the same time that he was disappointed in this expectation, he was surprised by a very unlooked─for phenomenon. All the pieces which he had thus attempted to melt, when they were afterwards exposed to the light, became themselves luminous. It is the singular property of the Bologna phosphorus, after it has undergone calcination in a particular manner, to become capable of imbibing so much light on exposure, for a little while, to the light of the sun, or even to the flame of a candle, that it will afterwards shine in the dark for an interval of from eight to fifteen minutes, like a glowing coal, but without any sensible heat. The light which it emits is sufficient to read by, provided the letters be placed near it. If well prepared, the stone will retain this extraordinary property for five or six years.

The preparation of it is thus conducted. Pieces of sulphat of barytes are made red hot, for a few minutes, in a covered crucible placed in the middle of a fire, and then left to cool. When cool, they are pounded in a stone mortar, and sifted. The powder thus formed is made into a paste with a little gum arabic, and divided into long cakes, or cylinders, each about a quarter of an inch thick. These pieces are dried in a moderate heat, and then, by degrees, are exposed to a more violent heat, among charcoal, in a wind furnace. As soon as the coals of the furnace are half consumed, it must be filled a second time, and the phosphorus must be left undisturbed. When the coals are quite consumed the ashes must be carefully blown off with a pair of bellows, and the phosphorus will be found at the bottom of the grate.

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CLASS II.—SALTS.

ORDER I.—EARTHY SALTS.

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ALUMINE FAMILY.

197. _ALUM is a substance of yellowish or greyish white colour, usually opaque, but sometimes transparent. When purified, it consists of slender, irregular, hair─shaped fibres, and has a sweetish, astringent taste._

The alum of commerce is an artificial production from the different kinds of stones which contain it. That called _Roman alum_, from its being procured from the neighbourhood of Rome, is usually considered preferable to the other sorts; but good alum of our own manufacture is equal to it in quality. The Levant, or _Roche alum_, is said to have had its name from the village of Rocca, the present Edessa, in Syria.

There is a famous alum mine at Tolfa, near Civita Vecchia, in Italy. The alum is obtained from this mine nearly in a pure state; and it is so extremely hard, that it can only be wrought by means of pickaxes, and gunpowder. At Solfatara, near Naples, and in other volcanic countries, an abundance of alum is found, in a state of efflorescence, from the lava.

The alum of our own country is manufactured from a kind of slaty stone which is found near Whitby, in Yorkshire. This manufactory was first established about the conclusion of the sixteenth century, by Sir Thomas Chaloner, who is supposed to have obtained his knowledge of the process, from the alum works which had then lately been introduced into Germany and Spain. The rock of _alum slate_, near Whitby, is supposed to be nearly twelve miles in extent: and affords an abundant supply of alum. The workmen tear open the rock; after which the different fragments are loosened, in the form of slaty leaves or plates, that are of a dark grey colour. To obtain the alum, a bed of fagots is formed from ten to twelve feet in depth. By the side of this a scaffold is erected, which enables the workmen to form a pile of mineral about fifty feet long, and forty feet high. While this pile is forming, the fagots are lighted. By the gradual operation of the heat, a calcination takes place, in consequence of which the alum is afterwards rendered capable of being more easily separated than it otherwise would be from the stone in which it was contained, and from other extraneous matters that are combined with it. After this, the mineral is washed in shallow vessels, so arranged that the water may be poured from one into the other. By this process the alum becomes suspended in the water, while all the earthy particles subside to the bottom. The next operation is to evaporate the water saturated with alum. This is done by boiling it in large leaden caldrons, fixed, on cast iron bars, over a furnace. As soon as the contents of the caldrons are brought to a proper state, they are drawn off into casks, where the alum concretes into a mass. The hoops are then taken off, and the alum is broken and left to dry; after which it is packed in casks for sale.

Alum is an article of indispensable importance to dyers, not only on account of its cleansing and opening the pores of the substances to be dyed, and thus rendering them fit to receive the colouring particles, but also from its more essential property of fixing the colours in such manner that they cannot afterwards be washed out. By tanners it is in great request for giving firmness to the skins after they have been rendered flaccid in the lime─pits. It is employed in the manufacture of paper, and by engravers, and other artists. In the making of candles, alum is added to the tallow, to render it glossy, and to give it greater firmness and consistence; and, mixed with cream, it aids the separation of butter. It has a tendency to retard ignition. Paper soaked in alum water does not easily take fire, and is thereby better fitted for the preservation of gunpowder. Such paper is likewise used in the whitening of silver, and the silvering of brass. It has been recommended that ladies’ muslin dresses should be dipped in a solution of this substance, for the purpose of rendering them less liable to catch fire. A solution of alum also retards the putrefaction of animal substances, and affords useful, as well as economical, means of preserving natural productions that are imported from foreign countries. Alum is frequently mixed with paste, to prevent its losing its tenacity by the absorption of moisture. It is asserted that bakers occasionally use it as an ingredient in bread, and that its presence may be discovered by thrusting a heated knife into a loaf before it is cold: if free from alum, scarcely any alteration will be visible on the blade, but if the contrary, the surface, when cool, will appear slightly covered with an incrustation of alum. A very important purpose to which alum may be applied is in the purifying and sweetening of water that has become fetid and unfit for use; from five to ten grains of burned alum, and double or treble that quantity of pounded charcoal, will correct the fetor of a gallon of water. Printers’ cushions, and the blocks used for the printing of calicos, are rubbed with burned alum to remove any greasiness, which otherwise would prevent the ink or colour from sticking. This substance is also occasionally employed by surgeons to stop the bleeding of small vessels, to corrode fungous or proud flesh, and for other purposes in medicine.

198. MAGNESIA FAMILY.

This is a family of minerals which comprehends all the combinations of magnesia with acids.

When freed from extraneous matters, magnesia is a powdery substance of limpid white colour.

199. _EPSOM SALTS, or SULPHAT OF MAGNESIA, consist of magnesia in conjunction with sulphuric acid (24)._

It is said that Epsom salts have been found in the Alps, and in Switzerland, under a powdery form, and sometimes even in masses, or a state of incrustation on stones and rocks. They are, however, chiefly found dissolved in mineral waters, and particularly in those at Epsom in Surrey, and Sedlitz in Bohemia. Their taste is bitter and unpleasant. So little are they affected by exposure to the air, that the Abbé Haüy kept some by him for more than twelve years without any sensible alteration.

These salts are much used in medicine, and are sometimes manufactured from the waters of Epsom (290) and Sedlitz (289), but more frequently, and in much greater abundance, from sea─water.

The _magnesia_ of the shops is prepared by dissolving Epsom salts in water, and adding to the solution half their weight of potash (205). The substance that sinks to the bottom is magnesia; and this, washed with a sufficient quantity of water and dried, has the appearance of a light, soft, and white powder, of insipid taste.

Magnesia is used in medicine, both in a simple state and when calcined or burned. It is also employed in some chemical processes; and is in considerable request in the manufacture of enamel and porcelain. If putrid water be agitated with a small quantity of magnesia, it will lose a considerable portion of its bad taste and smell.

200. SODA FAMILY.

Soda, like potash (205), is an extremely caustic alkali (42). It has a greyish white colour, and agrees exactly with potash (205) in taste, smell, and corrosive quality, but it is not so heavy.

In a mineral state soda has hitherto been found only in combination with some acid.

Common salt (202) is a compound of soda with muriatic acid (29).

The soda of commerce is obtained from sea─water; and from the ashes of different kinds of plants that grow on the sea─shores, but particularly from that called _salsola soda_, which is found in great abundance on the coasts of the southern parts of Europe; and from which it has its name. It is sometimes called _barilla_, from the salsola soda being so denominated in Spain.

This alkali is of essential use in the arts. When melted with flint or sand, it forms glass, and answers much better for this purpose than potash. In conjunction with oil and lime, it is employed in the manufacture of soap; and it is used as a substitute for soap in the cleaning and bleaching of linen, flannels, and worsted goods. If a weak solution of soda be poured into foul bottles, or casks in which wine has long been kept, it will cleanse them. It may also be successfully used for the cleansing of vessels in which milk has become acid. Saddles, bridles, or boot─tops, may be effectually cleaned by means of this liquor, and restored nearly to their original colour and appearance.

The art of _soap─boiling_ may easily be illustrated by the following experiment. Take a piece of quick─lime, slake it gradually by sprinkling on it a sufficient quantity of water. When it is completely slaked, add to it about twenty times its weight of water. To this mixture add two parts, by weight, of common subcarbonat of soda, previously dissolved in a sufficient quantity of water. Boil the whole for about half an hour, strain it through a cloth, and boil it till so much of the water is evaporated that a phial that will contain an ounce of water will hold one ounce, seven pennyweights and a half, of this ley. Then mix in an earthenware pipkin or basin, one part of the ley, with two parts of olive oil. Place the mixture in a gentle heat, capable only of making the liquor simmer, and allow it to simmer, stirring the liquor continually, with a wooden stick, till, by letting a few drops of it fall on a plate, the soap will be found to coagulate, and the water become speedily separated from it. After which, pour out the contents into a cup, and suffer it to cool.—Soap may also be prepared without heat. If one part of the ley be mixed with two parts of olive oil, in a glass or stone ware vessel, and the mixture be stirred, from time to time, with a wooden spoon or spatula, it will become thick, and white; in seven or eight days afterwards the combination will be completed, and a white and firm soap will be obtained.

_White soap_ is formed of ingredients similar to those that have just been mentioned. _Yellow soap_ is made with tallow, resin, and soda. Soap may be formed by boiling shreds of woollen cloth with ley till the whole has acquired a certain consistence. This kind of soap has been made, and applied with success, in several manufactories in France.—The combination of oil and other ingredients with potash (205), instead of soda, affords what is called _soft soap_.

201. _NATRON, or CARBONAT of SODA, is a salt which consists of soda (200) in combination with carbonic acid (26). It is massive, of greyish colour, soluble in water, and has a disagreeable alkaline taste._

This salt is found in Egypt, on the surface of the earth, and particularly near the margins of certain lakes called natron lakes. In the summer season the water of these lakes is evaporated by the heat of the sun, leaving a bed of natron generally about two feet in thickness. This is broken with wedges and hammers; and packed up for sale in the European markets. The waters of some of the lakes contain both common salt and natron; and these, on evaporation, crystallize in successive beds. Natron is found in considerable quantity under the form of an efflorescence, on the surface of the earth, in the plains of Debreczin in Hungary. It is likewise found in small quantity in the ashes of most vegetables, but particularly in those of _salsoda_ and _salicornia_.

The ancient Egyptians are said to have made great use of natron for the preservation of dead bodies, by macerating them in it for several months previously to their being embalmed. Large quantities of this salt are sometimes imported into England, by the East India Company’s ships, from China, and other parts of the East. It is employed in the manufacture of soap, and for the washing of linen. Glass─makers mix it with sand for the formation of glass. On the continent it is administered as a medicine in complaints of the bowels and liver. The ancients sometimes employed a mixture of natron for soaking their seed corn, under an impression, that, when afterwards committed to the earth, it would thereby be rendered more fertile.

202. _COMMON SALT, or MURIAT of SODA, though found in some countries in a solid and massive state, is for the most part an artificial preparation from sea─water, and from the water of salt lakes and brine springs. It consists of soda (200) in combination with muriatic acid (29)._

Few productions, either natural or artificial, are in so much request as common salt. It is used by the inhabitants of nearly all countries, for correcting the insipidity of food. When applied in small quantities, it accelerates the putrid fermentation; and, in this case, is considered to aid digestion, by promoting the decomposition of the aliments. In larger quantity it has a contrary effect, and tends to preserve organic substances from corruption. Salt is used for glazing the surface of coarse earthenware; and is employed in several processes of dyeing.

When this substance is dug out of the earth it has the appellation of _rock salt_: and immense masses of it are found in different countries of the world. The most considerable, as well as the most celebrated _salt mines_, with which we are acquainted, are those about five miles from Cracow, in Poland; and it is supposed that they contain more salt than would be sufficient to supply the wants of the whole world for several thousand years. On descending to the bottom of these mines, a stranger is astonished to find a kind of subterraneous republic, consisting of many families, who have their own peculiar laws and polity. Here are likewise public roads, and carriages, horses being employed to draw the salt to the mouths of the mine, where it is taken up by engines. The horses, when once they are down, never more see the light of day; and many of the people seem buried alive in this immense abyss. Some are born there, and never stir out; others, however, have occasional opportunities of breathing the fresh air in the fields, and enjoying the light of the sun. The subterraneous passages or galleries are very spacious; and, in many of them, chapels are hewn out of the salt. In these are set up crucifixes, and the images of saints, before which lights are kept continually burning. In some parts of the mine huge columns of salt are left standing to support the rock. Its windings are so numerous and intricate, that workmen have frequently lost their way: the lights they carried have been burned out, and they have perished before they could be found. The salt is taken from these mines in blocks so large as, sometimes, to measure nine feet in length, four feet in width, and two or three feet in thickness. In the year 1780, the greatest depth to which the workmen had penetrated was about 320 yards, and the mass of salt was considered to be in some places more than 240 yards thick, and to extend at least three leagues.

Near the town of Cardona, about fifty miles northwest of Barcelona, in Spain, there is a mountain of salt, without cleft or crevice, 500 feet high, and nearly three miles in circumference. In the province of Lahore, in Hindostan, travellers have described a mountain of the same mineral, not inferior to this in magnitude; and the elevated regions of Peru afford rock salt at the height of 7000 feet above the level of the sea.

At Northwich and Nantwich, in the county of Chester, there are salt mines of great depth and extent. These are frequently visited by travellers, and are found amply to repay the trouble and inconvenience of descending into them. There are two principal beds of this substance; the upper one is about forty─two yards below the surface, and twenty─six yards thick. This was originally discovered about a century and a half ago, in searching for coal. The lower bed has already been examined to the depth of forty yards, without coming to the bottom; and it is about the centre of this bed that the purest salt has been discovered. The average depth of the cavity, formed by the workmen along the vein of salt in the different mines, is supposed to be about sixteen feet. In some of the mines, where pillars six or eight yards square are left to support the roof, the appearance of the cavity is singularly beautiful: and the effect is greatly increased when the mine is illuminated by numerous candles fixed to the side of the rock. The scene so formed would almost seem to realize the notion of the magic palaces of Eastern poets. Some of the mines are worked in aisles or streets. The methods employed in working out the salt offer nothing worthy of notice. Larger masses are separated from the body of the rock, by blasting with gunpowder; and are afterwards broken down with pickaxes, hammers, and other instruments. The present number of mines in the vicinity of Northwich is eleven or twelve, from which there are raised, on an average, 50,000 or 60,000 tons of salt per annum. The greater part of this quantity is exported to Ireland and the Baltic; and the remainder is employed in Cheshire, and the adjacent counties.

Salt is also made from _brine springs_ in Cheshire, Cumberland, Staffordshire, and Worcestershire; but the kind most commonly used in England is that which is made from sea water, and has the name of _sea salt_. The mode of manufacturing it is very simple. The water is first pumped into shallow reservoirs of earth, called salt pans, or salterns. In these it remains exposed to the sun until a certain proportion of the water is evaporated, so as to leave it about seven times stronger than in its original state. It is then conducted by another pump into flat iron pans, eight or nine feet square, and as many inches deep. These, being placed over a hot fire, the liquor or brine is boiled until nearly all the remaining particles of water have passed off by evaporation, and nothing is left in the pans but salt. This is thrown together into proper vessels, for a few days, to drain, after which it is fit for use.

In some countries the whole evaporation is performed by the heat of the sun; and, in extreme northern climates, where the sun would not have sufficient power for the operation, a very different process is adopted. The water is suffered to freeze in the salterns, and that portion of it which continues uncongealed is so strongly saturated that it requires only a moderate heat to evaporate the remainder of the water, and to crystallize the salt.

_Bay salt_ is that which is produced from the evaporation of sea─water by the heat of the sun only.

The inhabitants of Cardona, in Spain, make of the rock salt in their neighbourhood various transparent articles, which they vend at a cheap rate. These, which consist of small altars, figures of saints, crosses, chandeliers, salt─cellars, &c. are as clear as crystal, and, to appearance, as lasting. They are chiefly purchased by strangers as curiosities, and are distributed over various parts of Spain and the south of France.

The decomposition of salt furnishes the _muriatic acid_ (29), or _spirit of salt_ of commerce. This liquid, which is much used in the arts, and is in great request by chemists, is prepared, for common purposes, by mixing one part of common salt with seven or eight parts of clay, and distilling the mixture; or by distilling common salt and spirit of vitriol or sulphuric acid (24), and receiving the product into a vessel containing water.

It has been discovered that muriatic acid, in a state of gas, is an excellent means of correcting putrid exhalations. In the year 1773, the cathedral church of Dijon was so much infected by the corruption of bodies which had been interred within its walls, that it was entirely deserted. The professor of Chemistry at Dijon having been applied to for assistance, placed, on a few burning coals, in the middle of the church, a glass vessel containing six pounds of common salt. Upon this he poured two pounds of sulphuric acid (24), precipitately withdrew, and shut all the doors. The gas soon filled the whole cathedral. After twelve hours the doors were thrown open, and a current of air was made to pass through to remove the gas, which had entirely destroyed every putrid odour.

The following has been recommended as an eligible mode of fumigating rooms for the prevention of infectious disorders. Take six drachms of powdered nitre (206), and six drachms of sulphuric acid (spirit of vitriol); and mix them in a tea─cup, by adding to the nitre one drachm at a time of the oil. During the preparation the cup must be placed on a piece of heated iron, and the mixture stirred with a tobacco pipe or piece of glass. As soon as the fumes arise, the cup must be moved about to different parts of the room or house that are to be fumigated.

203. _GLAUBER SALT, or SULPHAT of SODA, is a salt which consists of soda (200) in combination with sulphuric acid (24). It occurs in an efflorescent or powdery state, on the borders of salt lakes; or, more commonly, in a state of solution, in certain mineral waters._

This salt, which was originally discovered by a German chemist whose name was Glauber, has a nauseously bitter and saline taste. It is found, in an efflorescent state, on meadow ground at Eger, in Bohemia; and on the walls of old galleries in mines, at Grenoble, in France. It is also abundant in the ashes of some kinds of vegetables, especially of sea weeds. The waters of the Mediterranean yield a great proportion of it; and the Glauber salt used for commercial purposes is chiefly prepared from sea─water, or by decomposing common salt, in order to procure muriatic acid (29). It may also be obtained by saturating soda with sulphuric acid (24).

The use of this salt in medicine is well known; and, in some countries, it is employed as a substitute for soda (200), in the manufacture of white glass. It ought to be kept in well─corked bottles, as otherwise the crystals soon fall into powder.

The following is a pleasing experiment, which shows a singular and almost instantaneous crystallization of Glauber’s salt. Dissolve this salt by adding portions of it gradually to water kept boiling until the water will dissolve no more. Pour the solution, whilst boiling, into common medicine phials previously warmed, and immediately cork them. Set the phials in a quiet place without shaking them. The solution, when cool, will remain perfectly fluid till the cork is taken out; but the moment this is done, and the air is admitted, it will begin to crystallize on its upper surface, in fine satin─like crystals, which will shoot downward, like a dense white cloud. In this act so much heat becomes evolved as to make the phial feel sensibly warm to the hand. When the crystallization is complete, the whole mass generally becomes so solid, that, on inverting the bottle, not a drop of it will fall out. If the crystallization should not immediately ensue on opening the phial, this may instantly be effected by dropping into it a minute crystal of the same salt. The experiment may be exhibited any number of times afterwards, by merely placing the phial in boiling water, till the salt it contains be again completely liquefied; and letting it stand, as before, to cool.

204. _BORAX is a salt composed of boracic acid (28) and soda (200), and is imported chiefly from the East Indies, in the form of a brownish grey, impure, shapeless salt, of sweetish taste; or in detached prismatic crystals, each about an inch in length._

Although borax has long been known as an article of traffic, there is scarcely any production with the origin of which we have been, till lately, less acquainted. It is found in a native, though impure state, in a mountain lake, situated about fifteen days’ journey from the capital of Thibet in the East Indies. This lake is so encompassed with hills as to have no stream either falling into it or flowing from it. The water is salt to the taste, and contains both borax and common salt; and the edges and shallow parts are covered with a stratum of this substance, which is dug up in considerable masses for exportation. It has here the name of _tinkal_, and is usually brought into Europe enveloped in a kind of fatty substance. The mode of refining it was for a long time kept, by the Dutch and Venetians, amongst those secrets which a want of sufficient research alone prevented from being generally known. When refined, it is called _borax_.

The uses of borax are numerous. It is employed as a flux for metals, being found to produce a more perfectly limpid fusion than any other substance. For the same reason it is made an ingredient in the finest kinds of glass, and particularly in some of the coloured glass pastes which are manufactured in imitation of gems. But its chief use is to jewellers and goldsmiths, to facilitate the soldering of gold and silver. Borax is also used in medicine.

205. POTASH FAMILY.

Potash is an alkaline substance (42), of white colour, and of smell somewhat resembling that which is perceived during the slaking of quick─lime (137). It is extremely corrosive, and remarkably acrid to the taste.

In a mineral state it is found only in combination with nitric acid (30).

Potash principally exists under the form of a salt, in vegetable substances; and is obtained by burning them, afterwards repeatedly washing the ashes with water, and then filtering and evaporating these to dryness. The appellation of potash was given to this salt from its having formerly been prepared in large iron pots.

The uses to which it is applied are numerous. In chemistry it is employed for a variety of purposes; and also in many arts and manufactures, in scouring, washing, bleaching, dyeing, glass─making, and several others. Its corrosive property is such that it is often used by surgeons under the name of _potential cautery_, to open abscesses, and to destroy useless or hurtful excrescences.

=Potash=, after it has been made red hot, is rendered whiter and more pure. In this state it has the name of pearl ash.

206. _NITRE, or SALTPETRE, is a salt which consists of potash in combination with nitric acid (30)._

_Its colour is whitish or limpid; and it does not liquefy by the action of the air. It is usually observed in the form of fine capillary crystals, though it is sometimes found in a massive state. When pure, it crystallizes into six─sided prisms (Pl. II, Fig. 15) which have a rectangular base. It is denominated by chemists_ nitrat of potash.

Nitre is found incrusted on the surface of the earth, in some parts of India, Africa, and Spain, and, in such abundance, as to admit of being swept off at certain seasons of the year, twice or three times a week. In our own country it not unfrequently occurs in a state of white efflorescence, on old plaster walls that are sheltered from rain. Nitre is also produced in stables and cart─houses, from the mixture of animal and vegetable substances in a state of putrescence.

Many kinds of plants, which grow in soils favourable to the production of it, contain nitre: this is particularly the case with pellitory, borage, and the large sunflower.